[perl #109726] PL_sv_undef loses identity

# New Ticket Created by  Zefram 
# Please include the string:  [perl #109726]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org:443/rt3/Ticket/Display.html?id=109726 >



This is a bug report for perl from zefram@fysh.org,
generated with the help of perlbug 1.39 running under perl 5.15.7.


-----------------------------------------------------------------
[Please describe your issue here]

Passing the standard undef object to a subroutine as an argument fails
to pass by reference.  Instead the sub sees a fresh *and writable*
undefined scalar:

$ perl -lwe 'BEGIN { *x = \undef; } sub mutate($) { print \$_[0], " second"; $_[0] = 456; } print \$x, " first"; mutate($x); print "done";'
SCALAR(0x8dc6500) first
SCALAR(0x8dc9758) second
done

whereas any other scalar has its identity preserved:

$ perl -lwe 'BEGIN { *x = \123; } sub mutate($) { print \$_[0], " second"; $_[0] = 456; } print \$x, " first"; mutate($x); print "done";'  
SCALAR(0x86739f8) first
SCALAR(0x86739f8) second
Modification of a read-only value attempted at -e line 1.
$ perl -lwe 'BEGIN { *x = \!1; } sub mutate($) { print \$_[0], " second"; $_[0] = 456; } print \$x, " first"; mutate($x); print "done";'
SCALAR(0x8d5e510) first
SCALAR(0x8d5e510) second
Modification of a read-only value attempted at -e line 1.

The latter case above is PL_sv_no, which is immortal in the same way as
the misbehaving PL_sv_undef.  Devel::Peek::Dump() confirms the identity
of these special scalars.  Attempting mutation without the intermediation
of a subroutine call consistently produces the expected failure:

$ perl -lwe 'BEGIN { *x = \undef; } $x = 456; print "done";'               
Modification of a read-only value attempted at -e line 1.
$ perl -lwe 'BEGIN { *x = \123; } $x = 456; print "done";'  
Modification of a read-only value attempted at -e line 1.
$ perl -lwe 'BEGIN { *x = \!1; } $x = 456; print "done";' 
Modification of a read-only value attempted at -e line 1.

[Please do not change anything below this line]
-----------------------------------------------------------------
---
Flags:
    category=core
    severity=low
---
Site configuration information for perl 5.15.7:

Configured by zefram at Sat Jan 21 18:33:38 GMT 2012.

Summary of my perl5 (revision 5 version 15 subversion 7) configuration:
   
  Platform:
    osname=linux, osvers=2.6.32-5-686, archname=i386-linux-thread-multi
    uname='linux vigo.rous.org 2.6.32-5-686 #1 smp thu nov 3 04:23:54 utc 2011 i686 gnulinux '
    config_args='-des -Darchname=i386-linux -Dcccdlflags=-fPIC -Dccdlflags=-rdynamic -Dprefix=/home/zefram/usr/perl/perl_install/perl-5.15.7-i32-f52 -Dman1ext=1 -Dman3ext=3perl -Duselargefiles -Dusethreads -Uafs -Ud_csh -Uusesfio -Uusenm -Duseshrplib -Dusedevel -Uversiononly -Ui_db'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=define, usemultiplicity=define
    useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
    use64bitint=undef, use64bitall=undef, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
    optimize='-O2',
    cppflags='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include'
    ccversion='', gccversion='4.4.5', gccosandvers=''
    intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12
    ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
    alignbytes=4, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -fstack-protector -L/usr/local/lib'
    libpth=/usr/local/lib /lib/../lib /usr/lib/../lib /lib /usr/lib /usr/lib64
    libs=-lnsl -lgdbm -ldl -lm -lcrypt -lutil -lpthread -lc -lgdbm_compat
    perllibs=-lnsl -ldl -lm -lcrypt -lutil -lpthread -lc
    libc=/lib/libc-2.11.2.so, so=so, useshrplib=true, libperl=libperl.so
    gnulibc_version='2.11.2'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-rdynamic -Wl,-rpath,/home/zefram/usr/perl/perl_install/perl-5.15.7-i32-f52/lib/5.15.7/i386-linux-thread-multi/CORE'
    cccdlflags='-fPIC', lddlflags='-shared -O2 -L/usr/local/lib -fstack-protector'

Locally applied patches:
    

---
@INC for perl 5.15.7:
    /home/zefram/usr/perl/perl_install/perl-5.15.7-i32-f52/lib/site_perl/5.15.7/i386-linux-thread-multi
    /home/zefram/usr/perl/perl_install/perl-5.15.7-i32-f52/lib/site_perl/5.15.7
    /home/zefram/usr/perl/perl_install/perl-5.15.7-i32-f52/lib/5.15.7/i386-linux-thread-multi
    /home/zefram/usr/perl/perl_install/perl-5.15.7-i32-f52/lib/5.15.7
    .

---
Environment for perl 5.15.7:
    HOME=/home/zefram
    LANG (unset)
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/zefram/usr/perl/perl_install/perl-5.15.7-i32-f52/bin:/home/zefram/usr/perl/util:/home/zefram/pub/i686-pc-linux-gnu/bin:/home/zefram/pub/common/bin:/usr/bin:/bin:/usr/local/bin:/usr/games
    PERL_BADLANG (unset)
    SHELL=/usr/bin/zsh

• [perl #109726] PL_sv_undef loses identity by Zefram
• Re: [perl #109726] PL_sv_undef loses identity by Dave Mitchell
• Re: [perl #109726] PL_sv_undef loses identity by Zefram